Formwork is a temporary structure that was erected for the purpose of allowing the wet concrete to be retained and shaped into desired shape and form. Its structure usually consists of sheathing, beams, walings, ties and any other supports that are necessary to ensure that the formwork structure is stable and rigid. It must also be safe for any worker working on or near the formwork.
In order to ensure the stability and rigidity of the formwork, it is important that sufficient longitudinal, transverse and diagonal bracings are provided. In addition, continuity of the structure, vertical supports and the structural conditions of the support members are also the key areas that must be looked at when designing and erecting formwork.
As for slab formwork, there is a proprietary formwork system called tableform which can limit or eliminate the need for site formwork erection. This can be done because tableform is a combination of soffit form and its supporting structure that are struck, moved and erected as one unit from one floor to another.
When considering the economic of construction, tableform is most likely used on high rise multi-stories structure with plain slab but also in low rise repetitive work. Construction details that most likely to hinder the use of tableforms are deep edge beams or other beams crossing in line of withdrawal of the tableform from the structure. Upstand beam or spandrel walls will further complicate matters and may need special adaption, such as folding legs.
The layout and size of the tableforms are usually determined by the shape of the soffit and the arrangement of columns, walls and access openings, together with the limitation of crane capacity of lifting gear. The leg loading and positioning will have to be considered in relation to the capacity of the supporting slab in the permanent structure, which may require back propping.
The design of tableform should follow the same principles as conventional soffit support system but the legs will be more widely spaced and therefore more heavily loaded. If adjustable steel props are used as part of the table framing, they are usually inverted and have a purpose-made bracing system.
Tableforms should normally be struck by first lowering the jack slightly, then releasing the form from the floor soffit to avoid shock load on the slab below. Only then should lifting devices be attached and the weight taken. Attempts to take the weight of the formwork while it is still in contact with the floor soffit can result in lifting devices becoming over-stressed and possible damage to the crane, formwork, falsework and structure.
The conventional tableform of consisting a lot of soffit parts is causing it to be a rigid construction structure that copious diligence is required to have it installed and dismantled. It is also a limitation that said tableform containing a fixed top surface being not applicable to be further used for another time casting another ceiling floor.